1. Field of the Invention
The present invention relates to vehicles in which two or more driven wheels are each connected to an individual separately excited DC motor yet driven by a single controller. More particularly, the present invention relates to methods and apparatus for detecting, eliminating and recovering from a condition in which a single wheel loses contact with the ground or spins on a low friction surface.
2. Description of the Related Art
Many electric vehicles have unique performance requirements that pose difficult obstacles to the design of control systems that are used to control the vehicles. For instance, aerial platforms (e.g., scissor lifts) and tuggers used to tow luggage within airports are typically difficult to maneuver due to the potentially large and uneven weight distribution, as well as the proportions of the vehicle. In order to increase the control with which these vehicles are operated and provide increased traction, separate motors are often used to independently drive wheels on the left and right sides of the vehicle.
In an electric motor vehicle controlled by two or more DC motors connected in series, there are many potentially undesirable environmental obstacles that can prevent normal operation of the vehicle. As one example, when the vehicle is driven into a pothole or over a ramp, one of the wheels may lose contact with the ground and spin, making it difficult if not impossible to steer the vehicle out of the pothole. As another example, driving over ice or other low friction surface may cause one of the wheels to spin uncontrollably. As the offending motor speeds up it produces increased back Electro-Motive Force (EMF). As the EMF increases, it robs the other motor of applied voltage. As a result, the motor current decreases and a total torque produced by both motors decreases. This often leads to a stalled vehicle or roll-back.
Drive systems of electric vehicles such as aerial platforms often employ dual motors with a single controller to save costs while providing increased traction. However, in those systems that do attempt to resolve the spinning wheel problem, the motors are connected in parallel or to separate controllers, increasing the system cost. Moreover, many such systems require that the operator detect the spinning wheel problem as well as manually initiate an anti-spin control mechanism to alleviate the spinning wheel problem. Accordingly, there are continuing efforts to provide improved control mechanisms that facilitate detecting, eliminating and recovering from a single wheel spin in a vehicle in which two or more driven wheels are each connected to an electric motor.
The present invention enables the performance of electric motors having armature and field coils that are independently excited by a source of voltage, in which the armature coils are connected in series, to be controlled by a single controller. Since each electric motor has armature and field coils that are independently excited by a single source of voltage, power around either the armature or field coils may be separately shunted. Using such a shunting mechanism, once a xe2x80x9csingle wheel spinxe2x80x9d is detected, recovery from the single wheel spin may be performed.
In accordance with one aspect of the invention, methods and apparatus for controlling performance of electric motors in a system having two or more electric motors are disclosed. The motors are adapted for being coupled to two or more wheels of an electric vehicle, where each of the electric motors include armature and field coils which are independently excited by a source of voltage to generate armature and field currents. In addition, the armatures are connected in series to the voltage source. The methods and apparatus include detecting a condition in the system indicating that one of the wheels is slipping, reducing power delivered to one of the motors that is associated with the slipping wheel in response to the detection of the condition, providing power to the one or more motors that are not associated with the slipping wheel after the power delivered to the motor associated with the slipping wheel is reduced, and restoring the power delivered to the one of the motors associated with the slipping wheel in response to a recovery event.
In accordance with another aspect of the invention, a condition in the system is detected to ascertain when to deactivate one of the motors. One of the motors to deactivate is identified when the condition is detected. Current in the identified motor is then eliminated. The current may be armature current or field current. Armature current and field current are then provided to the one or more motors that are not identified as one of the motors to deactivate after the current is eliminated in the identified motor. The current in the identified motor is then restored in response to a recovery event.
A recovery event may be defined by a variety of different events. For example, a recovery event may include the detection of movement of the vehicle in a desired direction, movement of the vehicle for a specified period of time, and/or movement of the vehicle for a specified distance. Similarly, a recovery event may be a lapse of a predetermined period of time or a user-initiated event such as the return of a throttle in the vehicle to neutral or initiation of braking of the vehicle.